I am investigating the genomic organization and developmental expression of the actin multigene family of the sea urchin, S. purpuratus. The first priority has been to isolate, by recombinant DNA techniques, a complete representation of the actin-encoding sequences present in this organism. These clones are providing the basis for an analysis of actin gene structure and topographical arrangement in sea urchin DNA, as characterized by restriction endonuclease mapping, electron microscopy and DNA sequencing. The data obtained thus far indicate a large (about 20 copies) gene family with diversity in intron organization, patterns of linkage and DNA sequence. A detailed comparison of different members of the actin gene set should therefore provide information relevant to the evolutionary mechanisms involved in gene duplication, divergence and fixation. Moreover, DNA sequence data generated in these studies will be directly relatable to protein structure, initially in terms of primary amino acid sequence conservation and potentially at the level of defining functional protein domains. The cloned actin sequences are also being used as probes to investigate the expression of particular actin genes during cell differentiation. RNA blot hybridizations, Sl nuclease mapping and in vitro translation studies will establish correspondences between actin mRNAs, the genes from which they are transcribed and the protein products which they encode. Through cell fractionations of developing embryos, this information can be extended to identify tissue-specific patterns of gene activity. Once individual actin gene transcription is determined, this data can be related to DNA sequence organization and the changes in DNA or chromatin structure which may accompany gene activation during development.